The general aim of this proposal is to develop better understanding of the unique problems of high resolution Position Emission Tomography (PET) and to further its development by characterizing and evaluating performance of the new clinical systems, developing a very high resolution system to serve both as a system for instrumentation development and for development of tracer kinetic models through its qpplication in animal research studies, and developing solid state photodetector technology to further improve resolution and eventually reduce system costs. Specifically, the proposal is divided into three subprojects: 1. Systematic study of problems unique to high resolution PET on current state of the art systems. Comprehensive evaluation of current high resolution systems will be performed to ascertain design weaknesses, to develop techniques to optimize signal to noise ratio in images, and to develop data correction techniques to allow extraction of quantitative information from PET images. Areas to be studied are attenuation correction techniques, normalization and calibration techniques, utilization of interdetector crosstalk events, deadtime correction, correction for accidental and scatter coincidences, and compensation for spatial distortion. 2. Development of very high resolution PET system for physics and animal research. A detection system will be developed based on scintillation detectors, photomultipliers (PMTs) and an analog encoding logic. System will be compatible with previously developed data collection and analysis system, minimizing development time. New system will serve to develop very high resolution imaging techniques, act as test system for detector development program and provide data base for tracer kinetic modelling through its use in animal research studies. 3. Development of silicon avalanche photodiode (APD) to replace or supplement PMT in detection system. Beveled edge type APD will be developed for use with scintillation detectors with emphasis on optimizing fast timing characteristics. Initial development will concentrate on optimizing single photodiode-scintillation detectors. Long term goal is multielement APD-photodiode capable of providing modular detection system for PET of higher resolution and lower cost than is currently possible.